_Relativity._ Dependence of one quantity or knowledge upon another. The Theory of Relativity formulated by Albert Einstein, based upon the principle that the universe is a four-dimensional (time-space) continuum, includes the conclusions that there is no absolute length, that unobstructed light rays have a constant velocity irrespective of the relative velocity between the observer and the source of light, that the velocity of matter can never reach the velocity of light unless the matter becomes light, that the mass of a body depends on its velocity. _Reproduction._ Generation, the process of creating offspring of about the same kind as the parents. Apparently the power of reproduction is that which chiefly sets apart the living from the non-living. However, the fact that an individual organism is incapable of reproducing or taking part in reproduction does not show that it is dead. _Reptile._ One of a class (Reptilia) of animals including snakes and crocodiles. The Reptilia are vertebrate animals which breathe air. _Resistance._ Non-conductivity, opposition. _Rheo-_, Stream, Current-, in compounds, chiefly electrical terms. As rheostat: a device for regulating current by controlling resistance. _Rhizo-_, Root-, in compounds. As, rhizopod: one of a class (Rhizopoda) of protozoa having psuedopodia (“false feet”) which resemble roots. _Rhodium._ A metallic element. Symbol: Rh. _Rodent._ One of an order (Rodentia) of mammals, including rats, rabbits, squirrels, and other gnawing animals. _Röntgen rays_ or _X-rays_. Forms of radiation produced when cathode rays strike the walls of the tube or the surface of a body placed within the tube. _Root._ A part of a plant, not necessarily but usually growing underground, which attaches it to supporting points and conveys nourishment. _Rubidium._ A metallic element. Symbol: Rb. _Ruminant._ One of a division (Ruminantia) of cud-chewing or at least herbivorous mammals with hooves. _Ruthenium._ A metallic element. Symbol: Ru. _Salt._ A compound in which all or part of the hydrogen of an acid has been replaced by a metal, or by some other inorganic element or radical. Common salt is one of the general class of salts, and it is known to chemists as sodium chloride. _Samarium._ A metallic element. Symbol: Sm. _Saponify._ To turn a fat or oil into soap by combining it with an alkali. The term also includes a wider range of reactions in which hydrolysis is involved. _Saturate._ To cause a substance to combine with or to absorb the greatest possible amount of another substance. To neutralize. _Scandium._ A metallic element. Symbol: Sc. _Science._ Exact, organized, and usually generalized knowledge. A special scientific field or system. Specifically, physical or natural science, not including philology or sociology. It is best not to attach too much importance to such a distinction. See also Art and Common sense.